Does it have cost estimation ?

I want an autoresponder that when a subscriber input an amount , the respond will auto deduct a 20% off to that amount posted.

For example , when the amount is 100,000$ the autoresponder will reply 80,000 offer instead .

Is this somehow possible ?

The form that they need to fill up is something they have to sell and I want them to offer a 20% off the price to buy the item .

Thanks.